Uh. The main thing that makes people fat is immobility... or glandular disorders, chemical imbalances, bacteria or viruses. Soda, in and of itself, while otherwise terrible for you (unless we're just talking seltzer with a twist) will not make you fat without the presence of the other issues I mentioned - lack of exercise or biology. When these are present anything a person eats can bring on weight gain.
That said... soda doesn't make people fat. People (and their biology) make people fat. Why not just drink water? Throw some ice and a slice of lemon, lime, orange, or cucumber in it. Yum-O! Cool, refreshing, thirst-quenching, and will in no way contribute to weight gain... in fact, a hydrated person's metabolic rate is a bit faster than a dehydrated person's... so is great for weight maintenance and loss.